As a method for sensing a trace substance, a sensing instrument using a quartz resonator has been known. This sensing instrument has a quartz sensor in which an adsorption layer for adsorbing a substance to be sensed is formed on a front surface of the quartz resonator, and measures the presence/absence or concentration of the substance to be sensed by utilizing the fact that when the substance to be sensed is adsorbed by the quartz resonator, more particularly, the adsorption layer, its natural frequency changes according to an adsorption amount of the substance to be sensed, and this sensing instrument is advantageous in that it is applicable to a wide range and has a simple structure as an instrument, and moreover, is capable of measuring even an extremely minute amount of substance because of its high sensitivity. For example, a patent document 1 describes that the use of a quartz sensor in the analysis of a disease marker substance contained in blood, urine, and the like is an effective method alternative to an immuno-latex kit which requires an expensive, large autoanalyzer.
The present inventor has been studying the possibility of applying a quartz sensor to, for example, dioxin and PCB which are environmental pollutants, a disease marker in blood, or the like, and the achievement in high-precision measurement of a target substance by this method would be innovative. The reason is because a method using a gas-chromatography mass spectrometer and an ELISA method (enzyme-linked immunosorbent assay method), which are currently known as methods of measuring, for example, dioxin, have the following problems. The former requires an extremely high instrument cost and thus a considerably high analysis cost, and takes a long period of time for analysis, and the latter is low in analysis precision, though requiring less instrument cost and analysis cost and taking a shorter period of time for analysis compared with the gas-chromatography mass analyzer.
A major part of a sensing instrument using a quartz resonator is structured such that an adsorption layer adsorbing a substance to be sensed, which has, for example, an antibody causing an antibody-antigen reaction is formed on the quartz resonator, an oscillator circuit is connected to the quartz resonator, and a frequency measuring unit measuring an oscillation frequency of the oscillator circuit is provided. After experiments on various kinds of substances to be sensed were repeated in order to determine an appropriate range of the oscillation frequency of the quartz resonator, it has been found out that even a frequency range appropriate for some kind of substance to be sensed is not sometimes appropriate for another substance to be sensed, and therefore, instead of fixing the oscillation frequency of the quartz resonator to one frequency, an actual product is preferably structured to be capable of selecting the oscillation frequency from a plurality of oscillation frequencies, for example, from at least two oscillation frequencies.
Further, a patent document 2 describes that, instead of measuring the oscillation frequency of a quartz resonator itself, calculating its difference from a reference frequency to measure a frequency change in the difference frequency is an advantageous method of measuring the frequency because a far lower frequency than the oscillation frequency is a target of this measurement. The patent document 2 further describes a structure in which sensor resonators different in oscillation frequency are used, differences between the oscillation frequencies fr1 to frn of the respective sensor resonators and reference frequencies f1 to fn corresponding to the oscillation frequencies fr1 to frn respectively are detected by a sampling circuit, and the detected difference of a channel selected by a selector is taken into a calculating device
Depending on the application of the sensing instrument of this type, it is sometimes preferable to separate an oscillator circuit-side unit to which the quartz sensor is inserted and an instrument main body including a measuring system and make them attachable/detachable to/from each other via a cable. However, in such an instrument, it is difficult to adopt the measuring method using the differences between the oscillation frequencies and the reference frequencies. The reason is that the reference frequency has to be selected according to the oscillation frequency of the used quartz sensor, but the oscillation frequencies are assigned individually to the respective oscillator circuit units, and in an instrument in which the oscillation frequency is changed by the replacement of the oscillator circuit unit, the instrument main body side does not know which of the oscillation frequencies is used. Such a problem is overcome if the oscillator circuit units and the instrument main bodies are in one-to-one correspondence, but this cannot be said to be a realistic product in view of cost and space.
Patent document 1    Japanese Patent Application Laid-open No. 2001-83154: paragraphs 0002, 0004
Patent document 2    Japanese Patent Application Laid-open No. Hei 6-241972